This invention relates to a method of producing a monolithic catalyst which is effective in simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NO.sub.x) in exhaust gases of internal combustion engines, the catalyst being of the type comprising at least one kind of noble metal of the platinum group deposited on an active alumina base coating provided to a monolithic carrier.
Various catalysts and methods of producing them have been proposed and put into practical use for the purification of exhaust gases of internal combustion engines mounted on vehicles such as automobiles. Much interest has been attached to so-called three-way catalysts which are effective for simultaneous oxidation of CO and HC and reduction of NO.sub.x. Conventional three-way catalysts generally employ noble metals of the platinum group, and particularly platinum, palladium and/or rhodium, as the principal catalytic component. In the current automobile industry, monolithic catalysts are preferred to granular catalysts.
A fundamental way of producing a three-way catalyst of the monolithic type is to first provide an active alumina base coating to the surfaces of a monolithic carrier which is formed of a ceramic material and has a honeycomb structure and then deposit platinum, palladium and/or rhodium on the active alumina base coating. Usually the deposition of the selected noble metal(s) is accomplished by pyrolysis of a suitable compound of each noble metal applied to the coated carrier as an aqueous solution. With a view to improving the activity and/or durability of the catalyst, it is known to introduce cerium in oxide form into the active alumina powder used as the principal material of the aforementioned coating (e.g., U.S. Pat. No. 4,206,087). Also it has been proposed to form an oxide of cerium, titanium, zirconium or the like metal on the active alumina coating initially provided to the monolithic carrier by pyrolysis of a suitable cerium, titanium or zirconium compound applied as a solution and then deposit a noble metal on the composite oxide coating (e.g. U.S. Pat. No. 4,294,726).
However, known three-way catalysts still need to contain relatively large amounts of noble metal(s) for attaining sufficiently high conversions of CO, HC and NO.sub.x. Since noble metals of the platinum group are very expensive and limited in resources, it is unfavorable to consume large quantities of such metals in the exhaust gas purifying catalysts.